CPAP treatment of Obstructive Sleep Apnea (OSA) involves the delivery of a breathable gas (generally air) pressurised above atmospheric pressure to a patient's airways via a conduit and a mask. CPAP pressures of 4 cm H.sub.2 O to 22 cm H.sub.2 O are typically used for treatment of OSA, depending on patient requirements. Treatment pressures for assisted ventilation can range of up to 32 cm H.sub.2 O and beyond, again depending on patient requirements.
For either the treatment of OSA or the application of assisted ventilation or similar, the pressure of the gas delivered to patients can be constant level, bi-level (in synchronism with patient breathing) or auto setting in level. Throughout this specification reference to CPAP is intended to incorporate a reference to any one of, or combinations of, these forms of pressurised gas supply.
A disadvantage of existing CPAP gas supply apparatus, especially those used in hospitals and the like, is the danger of biological contamination and disease/virus/bacteria transfer. More particularly, there can be a significant reverse flow during heavy expiration and/or coughing and biological material exhaled by a patient can be deposited in the gas supply apparatus and transferred to another patient who uses the same machine. Further, a patient continually using the same machine can be re-infected by a prior condition.
Hitherto, CPAP apparatus have basically comprised a closed outer casing surrounding internal components. Components inside, and constituting part of, the gas flow path include the gas inlet, inlet filter, impeller, outlet muffler and gas outlet. Components outside the gas flow path include control electronics, power regulators and motor. As a result, present CPAP apparatus have a gas flow path that is extremely difficult to clean/sterilise without the time consuming dismantling and removal of all the "gas flow path" components. Further, without disassembly, common sterilisation procedures such as autoclaving will damage the circuit boards and other electrical components.
An attempt to solve the above problem involves incorporatine a bacteriological filter into the gas flow path adjacent the gas outlet to prevent biological material being forced back into the machine and any such material leaving the machine and being inhaled by the patient. However, these filters are expensive and add a significant resistance to the air path requiring larger and noisier fans and motors. A more restricted air path also represents a major difficulty to administering bi-level CPAP treatment due to the higher inspiratory air flow requirements of patients with advanced respiratory disease. Moreover, it is difficult to produce a biological filter which can trap very small biological particles such as viruses and spores that is still able to pass the required amount of air with an acceptable pressure drop, as decreasing filter pore size decreases hydraulic permeability.
It is an object of the present invention to substantially overcome or at least ameliorate one or more of the above prior art deficiencies.